In vertebroplasty, the clinician or surgeon seeks to treat a compression fracture of a vertebra by injecting a curable bone cement such as polymethylmethacrylate (PMMA) into the fracture. In a related procedure called kyphoplasty, the clinician or surgeon first inflates a balloon in the vertebra to create cavity, and then injects the curable bone cement into the cavity. Each procedure is considered to be a form of vertebral body augmentation (VBA).
In the typical PMMA cement, the surgeon mixes a powder component having PMMA particles with a liquid component having MMA monomer. These two components react over the space of about 10 minutes to provide a hardened acrylic cement. The powder and liquids components typically also respectively contain an initiator (such as BPO) and an accelerator (such as DMPT) to adjust the pace of the reaction.
One of the critical areas in VBA is the management of the rapidly curing PMMA cement. The cement typically passes through three phases during its cure: a mixing window (where the viscosity of the cement is too low to safely inject into a fractured vertebral body); a working time window (wherein the viscosity is sufficient high to safely inject but not too thick as to inhibit its injection; and a cure window (where the cement viscosity becomes too high to allow for its further injection). Typically, conventional PMMA cements have working times of less than 10 minutes.
U.S. Patent Application 2010-0168271 (Beyar) discloses PMMA cements engineered to have increased working times. In one embodiment, the PMMA powder component has a bimodal particle size distribution that takes advantage of the high wettability of the smaller particle sized PMMA. See FIG. 1. In use, the smaller particle size portion is wetted quickly by the MMA monomer to provide a quick rise in the viscosity of the cement sufficient to allow the surgeon to quickly begin injecting the cement into the vertebral body. The larger particle size portion is then more slowly wetted by the MMA monomer to provide a more gradual increase in viscosity.
In another embodiment, the PMMA powder component has a bimodal molecular weight distribution that takes advantage of the high wettability of the larger weight PMMA. In practice, the larger molecular weight portion reacts quickly with the MMA monomer to provide a quick rise in the viscosity of the cement sufficient to allow the surgeon to quickly begin injecting the cement into the vertebral body. The smaller weight portion is then slowly wetted by the MMA, inducing a more gradual rise in viscosity.
Despite the significant advance made by Beyar, there remains a need for increased working times in PMMA cements.